1. Field of the Invention
The present invention relates to a method of exhaust gas purification and an exhaust gas purification system having a catalyst unit carrying a NOx (nitrogen oxides) occlusion-reduction type catalyst that reduces and purifies NOx in exhaust gas from internal combustion engines.
2. Description of the Related Art
Various research and proposals have been made regarding NOx (nitrogen oxides) catalysts to reduce and remove NOx in exhaust gas from internal combustion engines such as diesel engines and certain types of gasoline engines and various combustion units. One of such catalysts is a NOx occlusion-reduction type catalyst, which is a catalyst for decreasing NOx from diesel engines. By using a catalyst unit carrying the NOx occlusion-reduction type catalyst, NOx in exhaust gas can be purified effectively.
This catalyst unit is constructed, having a monolith honeycomb 30M whose structure is as shown in FIG. 7. The monolith-honeycomb 30M, as shown in FIG. 8, is constructed by forming multiple of polygonal cells 30S on a support 31 that is a structural material made of cordierite or stainless steel. On the walls of the cells 30S, as shown in FIGS. 8 and 9, a porous catalyst coat layer 34, which is a catalyst carrying layer, made of alumina (Al2O3) or zeolite is provided. The catalyst coat layer 34 increases the contact surface area with exhaust gas. On the surface of the catalyst coat layer 34, there are carried precious metal (catalytically active metal) 32 and NOx occlusion material (NOx occlusion substance: NOx occlusion agent; NOx absorbent) 33. A catalytic function is provided by the construction described above.
FIGS. 10 and 11 show the configuration and occlusion-reduction mechanism of catalytic substances 32 and 33 on the surface of the carrying layer of the catalyst unit. In the catalyst unit, precious metal 32 having an oxidation function and NOx occlusion material 33 having a NOx occlusion function, are carried on the catalyst coat layer 34. The precious metal 32 is platinum (Pt) or the like. The NOx occlusion material 33 is made of some of alkaline metals such as potassium (K), sodium (Na), lithium (Li) and cesium (Cs), alkaline earth metals such as barium (Ba) and calcium (Ca) and rare-earth metals such as lanthanum (La) and yttrium (Y). Depending on the oxygen concentration in exhaust gas, the catalyst unit having the above described construction will perform the function of NOx occlusion or NOx release with purification of the released NOx.
As shown in FIG. 10, in normal diesel engines, lean-burn gasoline engines and the like, the exhaust gas contains oxygen (O2). In such cases where the air/fuel ratio of exhaust gas is in a lean air/fuel condition, nitrogen monoxide (NO) emitted from engines is oxidized into nitrogen dioxide (NO2) with oxygen contained in exhaust gas by the oxidation catalytic function of precious metal 32. Then, the nitrogen dioxide is occluded in the form of nitrate to the NOx occlusion material 33 such as barium having a NOx occlusion function, thus purifying NOx.
However, if the above reaction continues, the entire portion of the NOx occlusion material 33 having a NOx occlusion function will turn into nitrate, and will therefore lose the NOx occlusion function eventually. Therefore, exhaust gas with high fuel concentration (rich spike gas) is generated by changing the operating conditions of an engine or by injecting fuel into an exhaust passage, and then fed into the catalyst. The rich spike gas has no oxygen and a high carbon monoxide (CO) concentration with a high temperature.
Then, as shown in FIG. 11, in the rich air/fuel condition where there is no oxygen and a high concentration of carbon monoxide with a raised exhaust gas temperature, the nitrate formed after occluding NOx, releases nitrogen dioxide and returns to the original barium and the like. The released nitrogen dioxide, since no oxygen exists in the exhaust gas, is reduced by the oxidation function of carried precious metal 32, thus purifying the exhaust gas. That is, nitrogen dioxide is reduced into water (H2O), carbon dioxide (CO2) and nitrogen (N2) by using carbon monoxide, carbon hydride (HC) and hydrogen (H2) that are reductants in exhaust gas.
For the purpose described above, in an exhaust gas purification system having a catalyst unit carrying a NOx occlusion-reduction type catalyst, as described in Japanese Patent Application Kokai Publication No. 1994-336916, for example, the following regeneration operation is performed. When an estimated NOx occlusion amount reaches a NOx saturation amount, the air/fuel ratio in exhaust gas is made rich, thus decreasing the oxygen concentration in inflowing exhaust gas. That is, a rich control is performed for restoring a NOx occlusion capacity. The rich control causes occluded NOx to be released and the released NOx to be reduced with a precious metal catalyst.
However, in the conventional rich combustion control for regenerating the NOx occlusion capacity of the catalyst unit carrying a NOx occlusion-reduction type catalyst, the rich control is performed at the excess air ratio λ of 1.0 i.e. at the theoretical air/fuel ratio in the initial stage of rich combustion as shown in FIG. 12. At the beginning, the oxygen adsorbed onto the surface of the catalyst is released. The released oxygen consumes reductant in exhaust gas. Therefore, sufficient amount of reductant does not remain to reduce released NOx, so that the NOx reduction reaction is insufficiently activated. As a result, the NOx concentration at a catalyst outlet Cnoxex is remarkably higher compared to the NOx concentration at a catalyst inlet Cnoxin, and a large amount of unpurified NOx is released into the downstream of the catalyst unit. That is, the air/fuel ratio adjacent to the catalyst surface does not become rich because of the oxygen adsorbed on the catalyst surface. Therefore, NOx cannot be reduced and flows out. As a result, there is a problem that the overall performance of NOx purification is degraded.
In addition, in this regeneration control of the catalyst unit, the release of oxygen adsorbed onto the catalyst surface occurs more easily, compared to the release of NOx due to the decomposition of nitrate. As a result, in the initial stage of the regeneration control, NOx remain in a NOx occlusion agent at a high degree. This causes a problem that the restoration of a NOx occlusion capacity becomes insufficient, unless the regeneration control time is set, taking the above described issue into consideration.
In order to solve the problem, there is considered making the exhaust gas more fuel-rich. That can be attained by measuring the amount of oxygen adsorbed onto the catalyst unit in an experiment in advance and increasing the amount of reductant corresponding to the amount of oxygen released from the adsorbed oxygen.
An example close to this consideration is proposed in Japanese Patent Application Kokai Publication No. 2000-27677, which is an exhaust gas purification unit for a lean-burn internal combustion engine. In this exhaust gas purification unit, a catalyst unit having an oxygen (O2) storage function is placed in the upstream of a catalyst unit carrying a NOx occlusion-reduction type catalyst. The catalyst unit in the upstream works as a catalyst unit for startup time and its main purpose is to remove HC and CO components that are released in large amounts from an engine at startup. In this exhaust gas purification unit, the catalyst unit for startup time releases oxygen during a rich spike operation for regenerating the catalyst unit carrying a NOx occlusion-reduction type catalyst. By releasing oxygen, it is intended to solve the problem that unpurified NOx are flowed out toward the downstream of the catalyst unit in the initial stage of the regeneration.
This exhaust gas purification unit comprises a means for decreasing storage that makes an air/fuel ratio even richer than that during the rich spike operation for regenerating a NOx occlusion-reduction type catalyst by adding reductant to consume the entire amount of oxygen released from the catalyst unit in the upstream for startup time. The means for decreasing storage prevents the air/fuel ratio adjacent to a NOx occlusion-reduction type catalyst from becoming less rich than the theoretical air/fuel ratio in the initial stage of the rich spike operation, thus preventing NOx from being unpurified.
However, if the regeneration control of increasing the amount of reductant corresponding to the amount of released oxygen is employed, NOx continue to be released even after the completion of oxygen release. Because of this, the amounts of HC and CO that are reductant in exhaust gas become excessive, after oxygen that has been absorbed by an oxygen storage function, is released from the surface of a catalyst. As a result, there arises a problem that the exhaust gas is extremely deteriorated since HC and CO that have not been used for the reduction of NOx are flowed out in unpurified state toward the downstream of the catalyst unit carrying a NOx occlusion-reduction type catalyst.
That is, as shown in FIG. 13, when the more fuel-rich control whose excess air ratio λ is smaller than 1.0, i.e. whose air/fuel ratio is smaller than the theoretical air/fuel ratio is performed, the NOx concentration at a catalyst outlet Cnoxex in the initial stage becomes lower. However, the HC concentration at the catalyst outlet Chcex becomes extremely high since the consumption of reductant is decreased when oxygen is no longer released. As a result, after oxygen adsorbed on the surface of a catalyst is consumed, a large amount of HC flows out (or slips) unused toward the downstream of the catalyst unit.
Meanwhile, an exhaust gas purification unit for an internal combustion engine is proposed, for example, as described in Japanese Patent Application Kokai Publication No. 2002-188430. In this exhaust gas purification unit, the feedback control for the supply amount of reductant is performed by using an air/fuel ratio sensor placed in the downstream of a catalyst unit carrying a NOx occlusion-reduction type catalyst. In the initial stage of regeneration control, the feedback control is stopped during a predetermined period until the output value from the air/fuel ratio sensor reaches a certain predetermined value, i.e. during a period until oxygen (O2) storage effect converges. This prevents unnecessary supply of reductant caused by the oxygen occlusion function of a NOx occlusion material (NOx absorbent). At the same time, this prevents the deterioration of exhaust emissions caused by excessive supply of reductant and the useless consumption of reductant.
However, in this exhaust gas purification unit for an internal combustion engine, in the downstream of a catalyst unit, an air/fuel ratio is considered to exhibit a high apparent air/fuel ratio temporarily due to the release of oxygen caused by oxygen storage effect. Based on this consideration, the unit prevents the supply amount of reductant from exceeding the amount of reductant that should be supplied to the catalyst unit.
Accordingly, the consumption of reductant by released oxygen is not taken into consideration. As a result, there arises a problem that unpurified NOx cannot be prevented from flowing out toward the downstream of the catalyst unit in the initial stage of rich spike operation.